In an article entitle "Viabilities of Wavelength-Division-Multiplexing Transmission System Over an Optical Fiber Cable" published in IEEE Transactions on Communications, Vol. Com-26 No. 7 (July 1978) at pages 1082 to 1087, Tetsuya Miki and Hideki-Ishio put forward a Wavelength division multiplexing (WDM) system using light emitting diodes (LEDs) having respective wavelengths of 784 nanometers, 825 nanometers and 858 nanometers.
The three LEDs in Miki et al are independently modulated and, because bandwidth overlap between the LEDs causes some interchannel interference, interchannel interference cancellers are used to effect a reduction in noise so caused.
In another paper published in the magazine Applied Optics dated Apr 15, 1979 at pages 1253-1258 a similar system employing laser diodes was discussed by Koh-ichi Aoyama and Jun-ichiro Minowa. In this case five laser diodes having respective wavelengths of 810 nanometers, 830 nanometers, 850 nanometers, 870 nanometers and 890 nanometers were used.
As will be seen from these two systems laser diodes permit closer channel spacing than LEDs. This is because laser diodes have a spectrum half-width of less than one-tenth of the spectrum half-width of LEDs.
Considering LEDs more carefully now it will be noted that, say, an 850 nanometer LED produces its peak power at 850 nanometers nominally but this peak power point will vary with temperature and tolerancing by around plus or minus thirty nanometers. The bandwidth to the half-power point is about one hundred nanometers so with drift half power of a nominal 850 nanometer LED may extend anywhere in a range from around 730 nM up to 930 nM in a commercial device.
Since in WDM systems interfering signals need suppressing to about one-one thousandth power (that is thirty dB down) normal roll off separation for a successful system would require channel separations of about 350 nanometers, thus using two LEDs as an example of say 850 nM and 1200 nM centre wavelength.
Another problem with LEDs is maintaining the accuracy of their centre wavelengths. Thus even if the bandwidth and drift problems are overcome, manufacturing LEDs with specific bandwidths requires accurate control of the chemical mix from which they are made. Thus whilst it is possible to manufacture or select small quantities of LEDs to accurate centre wavelength requirements, reliable commercial production of LEDs with closely spaced centre wavelengths separated by say a few nanometers would require a different plant for each centre wavelength manufacturing more accurately than we currently know how.
Accordingly providing a WDM system with narrow channel separation for optical transmission is a major problem.
It is one object of the present invention to provide an optical transmission WDM system in which this problem has been overcome.
It is another object of the present invention to provide an optical transmission system which was a high radiance and efficiency, high degrees of optical isolation between wavelengths and which is rugged and compact.